Neonatal facilities of hospitals require, under current care standards, the provision of infant beds which will automatically maintain the baby's environment within acceptable temperature limits. This necessitates the use of heating elements with controls which will effectively regulate the heat delivered to the unit in response to sensors in the bed which monitor the temperature in the vicinity of the baby and, preferably, the actual temperature of the baby's skin. It is desirable that the controls for such warmers have, in order to reliably regulate the baby's temperature, a stable and accurate control, and one which will also be responsive to abnormal and potentially unsafe conditions which require an alarm or other response.
Two types of controls have been available to regulate the temperature of infant warmers. One is the proportional or analog control by which power to a heater is varied as a function of the difference between the actual sensed temperature and a temperature setpoint. The other is the on/off type control or switching control which alternately energizes the heater, usually at a single power level, and switches off the heater, depending on whether the actual temperature is either below or above a setpoint, or below a lower minimum setpoint or above an upper maximum setpoint.
In proportional control, the temperature is usually controlled by gradually reducing the heater output as the setpoint is approached and/or exceeded, eventually turning off the heater when a maximum temperature is reached. If the temperature range around the setpoint over which the heater output is varied is wide, a very stable temperature control can be achieved, at least where the ambient conditions are stable. This is known as a loose proportional control. However, the amount of heat required to maintain the setpoint is a variable depending on external conditions which affect the rate at which heat energy is lost from the bed or unit. Where ambient conditions vary, loose proportional control may equilibrate at a temperature other than the setpoint. The equilibrium temperature which such a control may produce can, depending on ambient conditions, equal any temperature in the range of temperature over which the heater power is being tapered. The conditions which affect the equilibrium temperature would include such ambient conditions as air movement (wind chill), the proximating of attending or visiting personnel, or the operation of building heating and air conditioning equipment. Thus, although proportional control is desirable for stability, it is undesirable for accuracy.
In ON-OFF type of control, the temperature is regulated by switching the heater to full power until the measured temperature exceeds the setpoint by a specified amount, then switching the heater off until the temperature drops below the setpoint by a specified amount. This type of control continually overshoots and undershoots the setpoint by the amount of hysteresis which is dependent on the heater power level and the heat loss rate of the unit. While this type of control at least brings the temperature completely to the set point, the temperature oscillates about the setpoint so that it is neither smooth nor stable.
Infant warmers of the prior art possess at least one of the disadvantages of the proportional or on/off control systems described above.
Furthermore, although most infant warmers have high and low temperature alarms, incidents of overheating still occur with prior art devices due to the temperature sensing probe being partially dislodged from the skin of the infant or misapplied in such a way that the probe senses a slightly lower temperature than the actual skin temperature. In addition, the probe can be completely off the infant but positioned in such a manner as to be heated by the heating element, which is often a radiant element, to the temperature setpoint when the temperature of the infant is actually above or below the setpoint. All radiant warmers with servo control are subject to this hazard. When this problem occurs, since the temperature sensed may be within an allowable range of values, no alarm would be activated and the heater output may be regulated at too high or too low a temperature. Alarm systems based on temperature rely on the skin probe to sense the correct skin temperature of the infant. If the probe is not installed correctly, the alarms may not respond to the proper alarm condition.
Also, many radiant infant warmers have a manual mode of operation for use in situations where it is impossible or difficult to use the skin probes. This may be the case in a labor-delivery room, for example. When operating in such a manual mode, the temperature display is normally still active and reads whatever temperature the probe may be sensing as it rests in the warmer bed. Some warmer beds have a slide switch to select between servo and manual modes. This presents a possible hazard condition in that the warmer may be in manual mode and displaying a temperature that is within the range of an infant's skin temperature. In fact, the skin probe may be attached to the infant while the nurse may not notice that the switch is in manual mode. Depending on the manual heat setting, the infant may receive too much or too little heat.
In some heater beds, the heater power is controlled by a thyristor semiconductor device commonly known as a triac. A common failure of thyristors is to become shorted or to turn on when they should be off. This may result from electrical disturbances on the power lines. When this occurs, the heater turns on and is uncontrollable. This is of course very hazardous.
Each of the problems described above are problems currently existing in the art. Accordingly, there exists a need in the art to overcome the problems.